While some oil may be extracted from crude oil deposits under intrinsic pressure, most oil must be pumped to the surface and, because of the viscosity of the crude oil, it is advantageous to increase the mobility thereof by injecting heat-carrying fluid into the deposit.
The recovery of oil thus can be accomplished by so-called primary and secondary methods which generally can recover about 35% on average of the crude oil contained in the deposit.
For this reason, it is common to provide so-called tertiary processes to increase the yield or product of an oil field.
Various chemical and physical principles are used in tertiary mobilization of crude oil. In one approach, steam is injected. The steam forms a heat carrier and displacement medium. The increase in temperature in the oil field reduces the viscosity of the crude oil and thus allows its flow or transport to the extraction well more readily. The injection of steam also has the advantage that it increases the pressure in the deposit and thus facilitates the displacement of crude oil to the surface and from the regions in which the steam is introduced.
To generate the steam which is injected into an oil field, it is customary to provide relatively small steam generating plants which are placed as close as possible to the injection well. Using insulated distribution pipes for the heated steam, the latter is delivered to a variety of injection wells generally located around the extraction well.
The distribution piping, even though insulated, should be as short as possible to minimize capital costs and heat losses.
Using special injection pipes, the steam can be introduced into the deposit and, for example, one can inject the steam through the same well from which oil is extracted or through wells remote from the extraction well. The injection systems which are used are generally also quite complicated, since they may require well casings of special design, insulated steam-supply pipes which are also referred to as tubings and specially insulated couplings between the tubings which may be provided with annular compartments between special means for maintaining the space between tubings and casings relatively dry, all designed so that the heat loss from the steam in its travel to the subterranean deposit is as low as possible.
These steam injection systems are not without disadvantages. A principal drawback is that the heat losses are practically unavailable not only in the distribution piping between the steam generator plant and the injection wells, but also in the injection tubings, the losses increasing in a greater than proportional way with the depth of the deposit and hence the length of the well.
The heating of the casing or well lining from the heat emitted by the steam injection ducts provides additional stress.
To accommodate the mechanical strain applied to the system, relatively expensive techniques must be used, e.g. the casing may have to be prestressed.
In general, the equipment of well with a steam injection duct is for more expensive and complicated than the usual well piping.
It appears, therefore, that the problems involved with steam injection as a tertiary method of crude oil mobilization are bound in large measure to the fact that heretofore the steam generating plant was required to be at grade level. Even greater problems may be encountered if the technique is to be used an ocean-pumping rigs and platforms, where space is at a premium and the provision of a steam generating plant on a platform and the use of insulated ducts can cause serious difficulties with respect to access and available space problems.